The invention relates to a piezoelectric actuator, especially for actuating control valves or injection valves in motor vehicles. The piezoelectric actuator includes an actuator body in the form of a multilayer laminate of stacked layers of piezoelectric material and intervening metallic or electrically conductive layers acting as electrodes, one bottom plate toward the valve and an opposed top plate being mounted on each of the end faces of the actuator body.
One such piezoelectric actuator is known from German Patent Disclosure DE 196 50 900 A1 of Robert Bosch GmbH. A piezoelectric multilayer actuator of this kind, embodied as a laminated stack of piezoelectric layers with intervening electrode layers, must be installed with mechanical prestressing, because otherwise it would rip apart upon being actuated.
In the aforementioned Patent DE 196 50 900 A1, it is proposed that a piezoelectric multilayer actuator body be glued on its face ends to a bottom plate and a top plate, respectively. The requisite prestressing is brought to bear by spring bands which rest on both sides longitudinally of the actuator body and brace the bottom plate and the top plate together. Since an adhesive layer always has a certain elasticity, the rigidity of the entire system is adversely affected by the adhesive bonds on the face ends of the actuator body. However, since the actuator body must be fixed at the bottom plate and the top plate, fixation techniques that overcome the disadvantage of the lack of rigidity of the overall system are needed.
It is an object of the invention to make a piezoelectric actuator possible in such a way that its actuator body can be fixed to the bottom plate and the opposed top plate without adhesive bonds.
In one essential aspect, a piezoelectric actuator according to the invention is distinguished in that the actuator body on each of its end faces has one piezoelectrically inactive cover layer, and that the connection faces of the bottom plate and the top plate which adjoin the end faces of the actuator body have fixation elements that grip the cover layers, for at least nonpositive fixation of the cover layers of the actuator body. The fixation elements that group the piezoelectrically inactive cover layers of the actuator body nonpositively and selectively also positively can, in one exemplary embodiment, be indentations that are each made in the bottom plate and the top plate in such a way that to establish the nonpositive engagement, they are adapted to the outer dimensions of the adjoining end face of the actuator body, or of its piezoelectrically inactive cover layer. Such indentations can advantageously be formed by stamping of the bottom plate and the top plate. Economical stamping of this kind also allows a square or rectangular contour of the indentation, so that advantageously piezoelectric actuators with a rectangular or square outer contour can engage the indentations with positive and nonpositive engagement.
Circular indentations can also be stamped into the bottom plate and the top plate for the fixation of an actuator body of circular outer contour, or if only nonpositive engagement is necessary, an actuator body of square outer contour can be affixed. A circular indentation grips the edges of an actuator body of square cross-section by positive engagement.
In an alternative exemplary embodiment, the fixation elements can each have one fixation ring, whose inside dimension is adapted to the outside dimensions of the adjoining end face or cover layer of the actuator body, and which is fixed at or on the connection face of the bottom plate and of the top plate. If such fixation rings are used, it is advantageous if the cover layers of the actuator body are somewhat thicker.
The fixation rings can also have various inside contours, such as a square or rectangular, or a circular, oval of elliptical inner contour. Fixation rings with a circular inner contour can also be used to fix an actuator body of square outer contour, if it is possible to dispense with a rotary fixation, and fixation rings with an oval or elliptical inner contour can be used to fix actuator bodies of rectangular outer contour, again if a rotary fixation can be dispensed with. In that case, the fixation rings with their inner contour engage the edges of actuator bodies of square or rectangular outer contour by nonpositive engagement.
Since the actual elastic clamping means for prestressing the multilayer actuator are not the subject of this application, they will not be described in further detail here. It will merely be noted that a multilayer actuator of this kind, instead of being prestressed with the spring bands used in the above-cited DE 196 50 900 A1 fastened laterally of the actuator body between the top plate and the bottom plate, can also be prestressed with cup springs or cylindrical helical springs that centrally engage the top plate or bottom plate.
The invention will be better understood and further objects and advantages thereof will become more apparent from the ensuing detailed description of two preferred embodiments taken in conjunction with the drawing.